Most of the microbes in our guts appear to remain stable for years, perhaps even most of our lives, researchers reported Thursday.

An analysis of the bacteria in the digestive systems of 37 healthy women over a period of about five years found, for the most part, little variation over time, says molecular biologist Jeffrey Gordon of the Washington University School of Medicine, who led the research. As decades-long internal companions, Gordon says, many microbes "are in a position to shape our lives, to promote our health or, in certain circumstances, contribute to risk for disease."

Scientists have known for a long time that we all carry around bacteria that help us digest our food. But they apparently do lots of other things for us too.

"These are cells that are important parts of ourselves," Gordon says. "And they contribute to our health."

There's always been one big question about the microbes, he says: "Once these communities are formed, how long do they endure? What is the stability in healthy individuals?"

To try to get a sense of that, Gordon and his colleagues developed a new type of "gut check": a genetic analysis Gordon calls "a bar code of life." The technique involves repeatedly analyzing all the variations in a particular bacterial gene. Because each strain of bacteria carries a slightly different form of the gene, the forms act almost like name tags or "bar codes" that identify which strains are present.

The method is "a way of classifying organisms represented in an individual's gut community in a moment of time and over time," Gordon says.

Being able to test gut microbes from time to time could eventually prove to be a useful part of a checkup, Gordon says. For example, in the current study, published in this week's issue of the journal Science, Gordon and his team found that when several women lost weight, the makeup of their gut bacteria slightly shifted (though the scientists couldn't tell which came first — the weight loss, or the bacterial shift).

"By looking at someone's intestine we could pretty much tell how much weight they had lost or gained without having to put them on a scale," says Jeremiah Faith of Mount Sinai Hospital in New York, who helped conduct the study.

Another intriguing finding was that people's microbes seem to run in families — much as genes do. The researchers found more similarities in the gut microbes of related women — such as sisters, or a mother and her daughter — than among women who were not related.

"For everyone that we checked we were able to identify strains of bacteria that were shared between related individuals, which suggests that [they] had these microbes for a long time because many of these [relatives] lived far apart from each other now," Faith says.

The finding corroborates earlier work suggesting that our microbial communities tend to form early in life, largely from microbes we get from our mothers and other close relatives when we are young.

"In the same way our genome defines who we are, one could say that the microbial populations that inhabit us define who we are," says Eric Pamer of Memorial Sloan-Kettering Cancer Center in New York.

Because all the women in the study were healthy, the researchers did not examine what happens to our microbes when we do things like take antibiotics or probiotics. Stay tuned for future research.

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Transcript

AUDIE CORNISH, HOST:

It's ALL THINGS CONSIDERED from NPR News. I'm Audie Cornish.

Here's some news to think about while you're enjoying that Fourth of July picnic. It's about your digestive system, specifically the microbes that live in your gut. Researchers have found that we carry the same collection of bacteria with us for most of our lives.

ROB STEIN, BYLINE: Scientists have known for a long time that we all have helpful bacteria and other microbes in our guts. They help us digest food, make vitamins, they may even help fight off infections and regulate our metabolisms.

But Jeffrey Gordon at Washington University in St. Louis says there's always been one big question about these microbes.

JEFFREY GORDON: The question is: Once these communities are formed, how long do they endure? What is the stability in healthy individuals?

STEIN: Are they the same microbes? Or do they come and go? To try to get a sense of that, Gordon and his colleagues developed a new way to identify our microbes, a genetic analysis Gordon calls a bar code of life. He used it to monitor the microbes in 37 healthy women, year in and year out, for about five years.

GORDON: Using this method, this bar code of life, it was a way of classifying organisms represented in a given individual's gut community, at a moment in time and over time.

STEIN: In this week's issue of the journal Science, the researchers report that the microbes seem to change very little, at least in healthy people. And based on what they found, the scientists think they can project the long-term composition of our microbial community.

GORDON: These microbes, likely, are with us for most of our lives and, as such, are in a position to shape our lives, to promote our health or, in certain circumstances, to contribute to risk for disease.

STEIN: Contribute to our risk for disease by getting out of whack somehow. That might play a role in all sorts of diseases, like inflammatory bowel disease, maybe even things like obesity, diabetes. And if that's the case, then testing our gut microbes from time to time, a new kind of gut check could be very useful. Gordon says it would sort of be like getting a blood test as part of an annual physical.

GORDON: An individual comes in and not only do we get blood chemistries, but we also examine the representation of different microbes in his or her gut. This will give us a more comprehensive view of their health status and give us a way of understanding how things are changing.

STEIN: In fact, the researchers have already been able to measure one key thing about someone's health just by looking at their microbes. Jeremiah Faith helped conduct the study.

JEREMIAH FAITH: By looking at someone's intestine, we could pretty accurately tell, over a length of time, how much weight they had lost or gained without having to put them on a scale.

STEIN: Another intriguing finding was that peoples' microbes seem to run in their families, sort of like genes.

FAITH: For everyone that we checked, we were able to identify strains of bacteria that were shared between related individuals, which suggests that you had these microbes for a long time, because many of these people, you know, they live far apart from each other now.

STEIN: All this seems to reinforce the idea that the microbes in our guts are really important. And Eric Pamer at Memorial Sloan-Kettering in New York says that means we should be paying a lot more attention to them.

ERIC PAMER: In the same way that our genome defines who we are, one could say that the microbial populations that inhabit us define who we are.

STEIN: What remains unclear is exactly what we might be doing that disturbs our microbes and what that means for who we are and how we're feeling.